1. Field of the Invention
The present invention relates generally to a rotary drum for securing a sheet material thereon under suction, and more particularly to such a rotary drum as used in combination with a recording apparatus.
2. Description of the Prior Art
A rotary suction drum, which is used to secure a photosensitive material such as film or paper, on its own peripheral surface is known in the art. Such drums are mainly employed in combination with a recording apparatus. The drum of this kind includes a hollow cylinder in the peripheral surface of which fine holes are provided, the cylinder being rotatably driven and communicating with a suction unit. Sheet material placed on the cylinder surface is secured thereat under suction provided through the holes of the cylinder surface.
Recently, in line with the increase in the size of the sheet material dealt with, such rotary drum are becoming large. However, it is often necessary to record on a half-size sheet of photosensitive material, because it is disadvantageous to use a full-size sheet.
To secure such a half-size photosensitive material sheet on the cylinder surface, one procedure is to: mount the half-size photosensitive material on the cylinder surface, and simultaneously cover the remaining area of the cylinder surface (i.e. the area not covered by the half-size photosensitive material) by another sheet of material, such as used film, which is fastened to the cylinder surface by means of adhesive tapes, whereby the half-size photosensitive material is fixedly held by suction on the cylinder surface. However, this practice is labor-consuming, and residues of adhesive are likely to remain on the cylinder surface and cause trouble.
An improvement of the rotary drum has been proposed as is shown in the Japanese Laid-Open Utility Model Publication No. 59-141344. The explanation on the publication is as follows, with reference to FIG. 8: A rotary drum 51 includes a number of fine holes 52 on the peripheral surface thereof and walls 53, 54 which define four chambers inside the drum 51. Each chamber communicates with a blower 55 through conduits 56 to which electromagnetic valves V.sub.1, V.sub.2 and V.sub.3 are respectively provided which are selectively operatable. Photosensitive materials of various size can be effectively mounted on the peripheral surface of the rotary drum.
However, such an improvement still involves some disadvantages, one of which is that the joining of the conduits 56 connected to the drum 51 involves complicated work, and another is that the suction rate in the conduits 56 is likely to be reduced because of the fact that the conduits run around the drum. As a result, it takes a long time before the differential pressure required hold a photosensitive material on the peripheral surface of the drum 51 is obtained. These problems reduce the operational efficiency, and make the apparatus unsuitable for automatic film loading onto the drum.